How to generate repeatable random sequences with rand-int

Question

I want to be able to generate repeatable numbers using rand in Clojure. (Specifically, I want results of calls to rand-nth or Incanter\'s sample

Answer 1

A clean way:

(ns designed.ly.rand)

(def ^:dynamic *rand* clojure.core/rand)

(defn rand-1
 ([]
   (*rand* 1))
 ([n]
   (*rand* n)))

(defmacro with-rand-seed
 "Sets seed for calls to random in body. Beware of lazy seqs!"
 [seed & body]
  `(let [g# (java.util.Random. ~seed)]
     (binding [*rand* #(* % (.nextFloat g#))]
      (with-redefs [rand rand-1]
        ~@body))))

It redefines rand within the scope. Example:

(with-rand-seed 9
  (rand 4)       ; => 2.9206461906433105
  (rand-int 10)) ; => 2

BTW. Beware of lazy seqs: http://kotka.de/blog/2009/11/Taming_the_Bound_Seq.html (Apparently, this link redirects to https without a trusted certificate so here's a link to a version at Web Archive: https://web.archive.org/web/20120505012701/http://kotka.de/blog/2009/11/Taming_the_Bound_Seq.html).